The invention concerns a method for the measurement of the concentration or the partial pressures of gases, particularly oxygen, in fluids as well as a gas sensor for its execution. The measurement of the concentration of oxygen or other gases is of major significance in various fields of engineering, such as in chemical processing, environmental and medical technology etc.
Traditionally, dissolved oxygen is measured galvanically, polarographically or potentiometrically in electrochemical measuring cells according to the Clark principle, meaning, the current is measured as a function of the time or of the potential or the potential in the zero current state in an electrolyte between a cathode and an anode.
In order to conduct oxygen into the measuring cell inner zone, a membrane is located which is permeable to oxygen and to gas, respectively, and this membrane separates the electrolyte in the measuring cell from the fluid to be measured.
The measurement presupposes the equilibrium setting of the partial pressures outside and inside the cell, and/or the concentrations corresponding with it by way of the Henry constant. As the electrochemical reaction consumes oxygen, a constant forward flow must be produced by stirring the measuring cell in the fluid. For this reason, many manufacturers offer mountable stirring mechanisms. Further methods are also known which compensate the oxygen consumption and, therefore, are only suitable for the measurement of low concentrations or/and in small volumes. The measuring values must be pressure-compensated, and corrected with regard to salinity and temperature.
The measurement is locally defined at the location of the sensor. Problems easily occur as a result of contamination at the relatively small membrane as well as air inclusions during the change of membranes.
There are other possibilities of oxygen measurement, for example, in the evaluation of the oxygen-concentration-dependent modulation of the phase radiant light received.
For the concentration measurement of other gases, gas-specific methods are known, insofar as recourse is not made to gas chromatography.
All methods mentioned require a complicated and sensitive sensor technology. For this reason appropriate measuring equipment is expensive and has only a low service life and/or requires high expenditure with regard to maintenance and calibration. For measurements under rough conditions, for example the measurement of oxygen partial pressure in low stratums, they are completely unsuitable.
In deeper stratums, such as in the ground water, gas measurement has not been possible up to the present time, the reason being that a measurement could only be carried out by means of a sample extraction. As the water is under a correspondingly high pressure and has a special temperature, degasification would occur at sample extraction and measurement under atmospheric conditions and no measuring values allocatable to the location of measurement would be obtained. Moreover, local measurement would seem problematic because it is not possible to estimate which disturbance affects the system during sample extraction.
The invention is based on the task assignment of stating a method and a suitable gas sensor of the category described in the introduction, which work in an uncomplicated manner and with simple means, which are sturdy and which lead to representative results for the structure under examination.
It was discovered with surprise that the properties of oxygen-permeable synthetic materials such as e.g. PTFE or other gas-specific permeable synthetic materials can be used for building up an oxygen and gas sensor, respectively, which allows the determination of the partial pressure or the concentration of the oxygen or other gases of a gas mixture by means of direct pressure measurement because the partial pressure to be measured within a random fluid causes an equivalent pressure in an enclosed vessel whose wall consists at least partially of such a synthetic material permeable to oxygen and/or gas.
PTFE (polytetrafluoroethylene) and PTFE-similar products such as Teflon, a copolymer of PTFE and  less than 5 mol-% PAVE (perfluoro alkyl vinyl ether), have a relatively high permeability for oxygen. At the same time, PTFE has an outstanding significance among the synthetic materials because of its enormous resistance and stability opposite random influences from its ambient surroundings. Adhesion and cohesion of the material are very low. Subsequently, the frictional forces also are reduced to a correspondingly low level. The surfaces remain clean.
PTFE has a further remarkable property. The permeability of the synthetic material, e.g. opposite the good water-soluble carbon dioxide and the nitrogen mainly prevailing in the atmosphere, lies two orders of magnitude below that of other synthetic materials such as, for example, HDPE (high density polyethylene).
The utilization of the properties
relatively high oxygen permeability and
negligible permeability opposite other liquids and gases
with the gas sensor built up according to the invention leads to the result that the partial pressure of the oxygen in a mixture of substances causes an equivalent partial pressure in a closed Teflon vessel and that this, independent of the absolute pressure existing in the vessel and only dependent on the oxygen partial pressure prevailing beforehand in the vessel, causes a change of the absolute pressure. This means that the measurement of the oxygen concentration can be reduced to a simple pressure measurement.
The method can be applied to any random fluid media.
The pressure is measured by means of, for example, measuring sensors which have longtime and overload stability but which are also almost to a randomly small degree dimensionable and are linear in a wide range, such sensors being on the basis of monocrystal silicium and having an accuracy of approximately 0.4%. With this, the possibility of an inexpensive and precise oxygen partial pressure measurement can be assumed where the concentration is determined, depending on the peripheral and initial conditions of the measurement, both from the rising speed of the pressure with known material properties of the applied synthetic material as well as from the geometry of the measuring cell orxe2x80x94without this knowledgexe2x80x94as a function of the absolute pressure difference.
A further variant exists in the measurement by means of differential pressure transmitters against a reference vessel which is subject to known conditions, e.g. atmospheric conditions, and which is otherwise configured as similar as possible to the measuring system, meaning, in particular with reference to material selection of the measuring vessel and its dimensions.
Water vapor also diffuses through the wall of the vessel, as well as further gases albeit with larger time constants. For this reason, a calibration is performed before a measurement is carried out again where the vessel is purged with nitrogen or a gas mixture with a known composition and/or air. With a tempering of the purging gas, a certain measuring temperature can, if required, be set simultaneously in the vessel.
The measuring cell structural arrangement and the measuring principle can be adapted to the individual measuring problem in question. In the simplest case, the gas sensor consists of a hollow body whose wall is at least partially formed by means of an oxygen-permeable membrane and in which a pressure probe is arranged. In addition to this point-type measurement (e.g., also in spherical-shaped geometry of the measuring cell), and with the assistance of a hose-type measuring arrangement, the integrating measurement along a line or, with the assistance of a hose network, the integrating measurement of a surface is also possible at all times. With this, an essential objective within monitoring task assignments in the environmentxe2x80x94the determination of oxygen in the ground air or in the ground water of heterogeneous systemsxe2x80x94is obtained in a simple and representative manner.
In general, only point-type and qualitative in situ gas monitoring has been possible up to the present. As already determined above, the isochore time-scanning of pressure change registered at the measuring sensor is the result of locally limited diffusion actions if and when the pressure equalization in a measuring hose is fast opposite the locally different equalization actions of the partial pressure. The pressure measurement in such a measuring hose therefore integrates by way of all locally limited partial pressure changes, where the substance to be measured is not in immediate contact with the measuring sensor, meaning, the pressure sensor. If a concentration gradient for oxygen to the gas mixture exists at any random contact location of the outer zone with the collecting line, it leads to an increase of the oxygen partial pressure inside the hose as a result of the oxygen diffusion. This partial pressure increase at the sample extraction location is measured at the measuring location as an overall pressure increase. The increased oxygen partial pressure does not have to exist at the measuring location, meaning, at the pressure sensor itself.
Therefore, a certain quantity of oxygen molecules does not have to exist at the location of the measuring sensor because the pressure and the pressure change are intensive state variables which propagate at sound velocity in the gas mixture in the internal zone of a hose and are therefore independent of the location within a closed system. The time resolution of the measuring systems is therefore determined only by the relatively slow diffusion kinetics into the measuring line.
For the in situ measurement the measuring vessel, e.g. a hose-shaped measuring line, can be solidly installed in the examination object and does not require any maintenance in this location.
A possibility for spatial limitation of the reference point(s) for the measuring is to make the collecting line gas-selective in certain areas only. The rest of the line can then be made of e.g. a high quality steel capillary.
The equation of state of the ideal gas p*V/T=const. applies, in good proximity, for gases in closed systems and under atmospheric pressure. At constant temperature, therefore, and instead of an isochore time-scanning of pressure change, the isobaric volume change or the product of pressure and volume change can be adopted for measurement. Both variables provide immediately the gas volume change as a function of time where the measurement under isobaric conditions is easier to interpret from a mathematical viewpoint. An additional gas-tight line can serve the purpose of determining the mean temperature.
If suitable collecting lines or networks are applied, at differential heights as required, to waste dumps, bioreactors and similar objects in order to measure, for example, oxygen distribution during aerobic degradation of organic pollutants, then the integral measuring principle is practically ideal for this application purpose.
A hose bundle with different materials provides a differentiated time behaviour of pressure or volume changes where the essential -and first change always occurs by the selectively preferred gas, however subordinating itself to the diffusion of the other gases in the ratio of the diffusion constants. A large number of such materials with different gas selectivity is known. For a hose number of the bundle which is equal to the number of the expected gases, and/or the gases to be measured, plus an additional hose as required, a simply solvable and certain equation system leads to the determination of representative partial pressure of the gases and the temperature in the ambient surroundings of the hose bundle which contains as material parameters the diffusion constants Dij and the temperature conductivity xcexj, the geometry of the hoses (length Lj, diameter 2Rj wall thickness bj of the hoses) and the measuring variables pressure pj (t) or volume Vj(t) (index I for gases, index j for hose).
A further application possibility are the area representative gas tracer tests for the purpose of determining the gas migration in the subsurface of the ground over parallel laid hose bundles for water-saturated and unsaturated conditions. In this case, for example, and by means of the combination of inert and reactive gases, the gas consumption as a result of microbic reaction or other gas consuming processes can be cumulatively determined and is available as a reliable variable for the thermodynamic interpretation of reactions in the subsurface.
The measuring principle is suitable for determining the oxygen concentration, also of further gases if required, and also in such locations where such a measurement had previously met with great problems, meaning, for example the direct measurement in the ground in greater depths or the direct measurement in waste waters and flowing waters.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.